Warn when non-monotonic timestamps are noticed

[devkev]

Since this generally indicates that things are in Bad Shape, and/or various assumptions made by mtools no longer hold.

[rueckstiess]

Perhaps a LogEvent could communicate back to the LogFile whenever its datetime is evaluated, should be reasonably cheap compared to the actual datetime parsing. As LogEvents are only acquired from LogFile objects now, and only in iterative sequence usually, that's probably the best place to put that warning.